Whether you are to use Relay outputs or Transistor (DC) outputs can depend on several factors.
1) Is leakage current going to present a problem of creating a false output? DC outputs tend to emit a small amount of current when off. If your devices are sensitive to this, then you should not use transistor outputs.
2) Current requirements - Most PLCs offer anywhere from 100 mA to 1 amp current driving for their transistor outputs (Entertron offers a controller with a 2 amp transistor output). Most relays for traditional PLCs are capable of approximately 2 amps, some offer higher ratings with fewer outputs (Entertron offers a minimum 7 amp / output ratings). If your current requirement to turn on your output device is higher than the PLC is capable of, you will require an additional relay to interface (solid state or mechanical) to interpose between the PLC and your device. This would also be required if your relay output rating was not sufficient to drive your device either.
3) Reliability - A transistor output will provide a longer life than a relay output due to no mechanical or moving parts. It is solid state. Keep in mind that the higher the inductive load, the shorter the life span, whether relay or transistor. Most PLCs rate their relay outputs for 1 million mechanical cycles (no load). The higher the inductive current, the lower the lifecycle rating.
When using transistor outputs, you will need to check and determine if the module you are using provides diode protection or if you will have to add it.
Remember that not all transistor or relay outputs are created equal. You should let your application requirements determine the controller and the capabilities you will need. Solid state reliability will cost more in the beginning, but over time will save in down time and aggravation.